The advantages of recording audio signals in a digital form (e.g., pulse code modulation, PCM) are well known. The bandwidth requirements of PCM recording, however, are substantially greater than that required for direct analog recording. This is due in part to the necessity for sampling the audio signal at the Nyquist rate and in part to the need for additional code bits for detection and correction of errors in the PCM code. Error correction is essential because digital systems tend to fail abruptly when errors occur in the bit stream.
The bandwidth needed for digital audio recording (e.g., 2-3 MHz) is within the capabilities of relatively inexpensive consumer or "home use" video disc and video tape systems. A digital audio signal may be made compatible with such equipment by recording it in a "pseudo-video" format, that is, one having a television-like line and field structure with simulated vertical and horizontal synchronizing signals. See, for example, U.S. Pat. No. 4,138,694 of Doi et al. Pseudo-video digital audio encoders and decoders for use with consumer and professional video tape recorders are commercially available. Examples are the Sony model PCM-1 for home use and the model PCM-1600 for professional use.
Video disc players have inherent advantages in playing back digital audio programs over video tape players, e.g., the ability to quickly access the recorded information, relatively low replication costs, relatively simple mechanical requirements, etc. This, unfortunately, is accompanied by a basic disadvantage relating to tracking errors. Specifically, tracking errors seldom occur in video tape recorders and a forward or backward skip of one track on a helical scan VTR results in only a one field error in the location of the pick-up transducer. A single skip of one track in a presently available video disc player (e.g., the RCA model SFT-100) results in an eight field error in the recovered signal. In another known player a skip of one track results in a two field error for disks recorded at a constant angular velocity.
It is known that one may correct tracking errors in a video disc player on a field-by-field basis thereby placing video disc players on a par with video tape players. An an example, in U.S. Pat. No. 4,313,134 of Rustman et al., a video disc system is disclosed wherein field numbers are recorded in pulse code modulation (PCM) form during the vertical interval of each video field. This has become known as a digital auxiliary information code (hereinafter DAXI code). Tracking errors (e.g., locked or skipped grooves or information tracks) are detected in a player by noting an improper sequence of two or more decoded DAXI field numbers. Correction of the error is provided by means of a transducer deflector (e.g., a stylus "kicker" solenoid or piezoelectric element) which radially translates the pick-up transducer in a direction to reduce the tracking error until such time as the field numbers again form an orderly sequence. The vertical blanking interval is also used in certain video tape recorder systems for conveying field identification signals (e.g., SMPTE or EBU time codes) to facilitate accurate field location for editing. See, for example, U.S. Pat. No. 4,159,480 of K. Tachi.